1. Field of the Invention
The present invention relates to a control device with a learning function, for driving a plurality of electric motors so as to move an object to be moved.
2. Description of the Related Art
In a machine such as a machine tool or an industrial machine, in the case that an object or a workpiece, having relatively low stiffness, such as a thin pin or a thin plate needs be moved, if a force for moving the workpiece is applied to a part of the workpiece which is far from the center of gravity of the workpiece, the workpiece may be bent or twisted, whereby it may be difficult to precisely move the workpiece. In such a case, by using two electric motors for moving the different parts of one workpiece, the workpiece is prevented from being bent or strained, whereby stable control may be possible. For example, when a cantilever-type workpiece having low stiffness is rotated and processed, the workpiece may be twisted due to acceleration or deceleration of torque and/or a machining load, whereby a precise rotational control of the workpiece cannot be realized. In such a case, torsion of the workpiece may be avoided by holding both ends of the workpiece, whereby precise rotational control may be realized.
However, depending on the configuration of the machine and/or the type of the workpiece, an imbalance may occur between each load applied to each electric motor. For example, in the case that the workpiece is rotated while both ends thereof are held, when a load applied to one electric motor becomes larger than a load applied to another electric motor, the response of the first electric motor is lowered and a delay occurs in acceleration/deceleration of the electric motor, whereby the workpiece may be twisted or vibrated. At this point, in order to equalize each response of each electric motor, it is necessary to adjust a speed gain of each electric motor corresponding to the type of the workpiece, as well as equalize a position gain of each electric motor. However, since there are a variety of workpieces such adjustment requires many man-hours.
In some cases, the types of electric motors for moving or rotating one workpiece are different. In such a case, the response of each electric motor may be different, whereby precise control cannot be realized.
Generally, in the case that an electric motor needs be precisely controlled based on periodically repeated commands for executing a periodic operation such as a reciprocating movement, a learning control is quite effective. For example, Japanese Unexamined Patent Publication No. 11-305839 discloses a control method for controlling a plurality of electric motors for moving one workpiece. In this method, a learning control is used for controlling one electric motor, and an amount of correction obtained by the learning control is used for controlling the other electric motors.
Japanese Unexamined Patent Publication No. 2007-42068 discloses a servo control device for executing synchronous control of two driving sources for driving two driving shafts, by which data for correcting a positional deviation of one driving source is calculated based on a synchronous error.
In the control method as described in Japanese Unexamined Patent Publication No. 11-305839, precise control may be possible when a workpiece has high stiffness. However, if the stiffness of the workpiece is relatively low, the workpiece may be easily bent or twisted. Therefore, if the amount of correction obtained by the learning control is used for controlling the other electric motors, precise control is not always possible. Further, when there is an imbalance between each load of each electric motor, even if the learning control as described in Japanese Unexamined Patent Publication No. 11-305839 is used, it is difficult to execute precise control because the response of each electric motor is different from each other.
In addition, the technique as described in Japanese Unexamined Patent Publication No. 2007-42068 is intended to reduce a synchronous error between a master side and a slave side, and therefore the technique cannot solve the above problem based on the difference between each response of the electric motors.